In semiconductor-based integrated photonics, a normal incidence photodetector (NIPD) measures light that is incident on the detector perpendicularly to the wafer plane, whereas a waveguide-based photodetector (WGPD) captures light that has been routed to the detector by in-plane waveguides fabricated on the wafer surface. To detect external optical signals, e.g., coming from an optical fiber, the external light is coupled, in the former case, directly into the NIPD, and, in the latter case, into an on-chip waveguide that is, in turn, coupled to the WGPD.
Integrated optical transceivers, e.g., as used in optical communications networks, often provide self-test functionality that allows testing the operation of one or more WGPDs within the receiver by routing light from an on-chip light source of the transmitter internally to the WGPDs, obviating the need to provide an external optical signal for testing. Beneficially, self-testing can greatly reduce test time and test cost, as well as improve manufacturability. However, during normal operation, when light is received from external sources, WGPDs, due to the difficulties in first coupling into a waveguide, suffer from higher insertion losses than NIPDs. Conversely, existing NIPDs are not suited for self-testing, as it is difficult to optically couple them to on-chip waveguides. Additionally, materials and fabrication methods differ between NIPDs and waveguides. Accordingly, in selecting photodetector types, PIC designers have been presented the choice between low insertion losses on the one hand and self-test functionality on the other hand, but have not been able to achieve both simultaneously.